In adjective clauses (subsentence that work like adjective = "que os parezca interesante" works like adjective of " otra época"...) we use subjunctive when you speak about something that you don't know if exists or not. You don't know if there are or not "otra época que os parece interesante". If you know, use indicativo: Voy a hablar de la otra que te parece interesante ( I know there are one that seems to you interesting ). Typical example: Busco un colega que HABLA francés (you know there is one) Busco un colega que HABLE francés ( you don't know...probably ther is one, probably not)

Depends on the context. Please, show the full sentence.

I don´t know how to explain it exactly, but the subjunctive is used to express an hypothetical situation. In your example: u otra que os parezca interesante = u otra que os pueda parecer interesante Is like a second conditional : If I were you I would go to the party --> Si yo FUERA tú, iría a la fiesta. In this case FUERA is a subjunctive. I'm Sorry, but I can't give you a rule of thumb for the use of subjunctive.